Modern indirect digital X-ray imagers typically rely on a scintillation screen to covert X-rays into visible light. The scintillation screen thickness typically depends on the X-ray energy being detected, higher energy X-ray energies necessitate thicker screens. However excessively thick screens have disadvantages. In particular, light spreading in thick phosphor screens decreases screen spatial resolution.
To minimize light spreading, scintillation screens are typically divided into cells. Such structures are described in U.S. Pat. No. 5,981,959 entitled “Pixelated Scintillation Layer and Structures Incorporating the Same” by Raj Apte and also _“Micro-electro-mechanical system fabrication technology applied to large area x-ray image sensor arrays”_by Daniel et. al. in the Journal of Vacuum Science Technology, A 19(4), 2001, Pages 1219-1223, both references hereby incorporated by reference in their entirety. In the prior art, cell walls are fabricated from SU-8 polymer (Microchem Corp.) using a photolithographic process. A reflective metal layer deposited over the cell walls renders the cell walls opaque. Reflective materials also help keep the light within the same cell. A phosphorous powder typically fills each cell. The phosphorous powder acts as a scintillation material that converts X-ray photons into light photons of energy in the visible range (e.g. green).
However fabricating the described structure is difficult and expensive. A major expense results from the difficulties associates with processing a thick photoresist. The high aspect ratio of the cells makes traditional molding techniques unsuitable for cell wall fabrication. Filling high aspect ratio cells without introducing voids has also proven difficult.
During use, the high aspect ratio cells exacerbate Swank noise problems. Swank noise results from the unequal propagation of light photons within the scintillator. In particular light generated in the conversion screen top layers do not contribute as much to the sensor signal as light generated in the conversion screen bottom layers. The effect, called Swank noise is described in an article entitled “Absorption and noise in x-ray phosphors” by R. Swank, J. of Applied Physics, Vol. 44, 1973, p 4199 and also more recently in “Effect of finite phosphor thickness on detective quantum efficiency” by Nishikawa et al., Medical Physics, 16 (5) 1989, p. 773.
Thus an improved method to form the X-ray scintillation screen and also a system for minimizing Swank noise effects is needed.